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Systems for data processing, anaylsis and representation

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Figure 2: The distribution of control points used in various tests
This makes the technique insensitive to image noise and
speckle. À final advantage is that the process is efficient,
taking only a few minutes of computer time.
The elevation model created by this technique is arbitrarily
scaled and may have a regional tilt. To provide surface
elevations, it is necessary to calibrate the elevation model
using measured surface heights. In this study, the model
was calibrated using the airborne altimetric data, which
allowed height values to be generated for regions of the
snow-field where there was sufficient surface detail for
successful stereo-matching.
The techniques and sources described above produced 15
control points, of which 9 had known elevation values.
These control points are of three types: surveyed points (1
point), planimetric points from the TM scene (6 points) and
heighted points from the shape-from-shading algorithm (8
points); the accuracy of the positions depends on their
source. The surveyed points have a nominal accuracy of
x11 m (Knight, 1986), but the local geometry of the survey
network greatly affects the accuracy of particular points.
Because the image is tied to the same survey network as
the surveyed points, the correspondence between the
image and surveyed points is within one pixel (i.e. within
30 m) and the nominal accuracy with which points on the
image can be located is x30 m. As the TM scene covers
a much larger extent than the study area, it was possible
to verify the co-location of the image with survey points not
used in this study. However, difficulties of identifying
corresponding points on the TM image and the aerial
photographs caused larger errors. Points on the edges of
rock outcrops were located more reliably than those at the
centre because of the high contrast between snow and
rock. Errors may also arise from the different dates of the
TM scene and the aerial photography. The extent of thin
snow patches may alter substantially from season to
season, and there is no means of quantifying this variation.